Next-near-neighbour interactions with Al in Li⁺- and Rb⁺-exchanged Na⁺ β-aluminas, detected by synchrotron X-ray absorption spectroscopy

Synchrotron X-ray absorption near-edge structure (XANES) spectroscopy studies have been carried out on the electronic and crystal structure environments around the Al atom in Na⁺ β-alumina and in two β-aluminas with Na⁺ exchanged by Li⁺ and Rb⁺. The aim is to define the type of interaction, if any, existing between the Al located in the `spinel block' and the fast-conducting cations in the `conduction plane'. Na⁺ β′′-alumina has also been studied for comparison. All β-alumina spectra differ from that of α-alumina (corundum) by showing additional features due to the presence of tetrahedral Al. Moreover, they all show a much greater degree of local disorder. There are definite, but small, interactions between tetrahedral Al (and, possibly, also octahedral Al) in the `spinel block' and the Na<sup>+</sup> and Rb<sup>+</sup> cations in the `conduction plane'; Na⁺ and Rb⁺ β-aluminas have similar Al K-edge XANES features, but with intensities that change in relation to the weight of the `conduction plane' atom. Despite differences in composition and structure, Na<sup>+</sup> β′′-alumina shows the same behaviour, thus confirming the substantial similarity of the Al local environments. Li<sup>+</sup>-exchanged β-alumina has an Al K-edge XANES spectrum that apparently differs from all others, but actually conveys the same basic information. Indeed, interaction between Al and Li is much greater than in any other β-alumina because Li<sup>+</sup> moves laterally off the `conduction plane' to become close to a facing tetrahedral Al, and strongly interacts with it. Thus, this study also draws attention to the fact that β-aluminas react differently to alkali exchange.